Operation of Molten Carbonate Fuel Cells With Enhanced CO2 Utilization

ABSTRACT

Molten carbonate fuel cells (MCFCs) are operated to provide enhanced CO 2  utilization. This can increase the effective amount of carbonate ion transport that is achieved. The enhanced CO 2  utilization is enabled in part by operating an MCFC under conditions that cause transport of alternative ions across the electrolyte. The amount of alternative ion transport that occurs during enhanced CO 2  utilization can be mitigated by using a more acidic electrolyte.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Patent Application No.62/773,438, filed Nov. 30, 2018 and entitled “Operation of MoltenCarbonate Fuel Cells with Enhanced CO₂ Utilization.” The entirety of theaforementioned application is incorporated by reference herein.

FIELD

Systems and methods are provided for operating molten carbonate fuelcells for enhanced CO₂ utilization.

BACKGROUND

This application discloses and claims subject matter made as a result ofactivities within the scope of a joint research agreement betweenExxonMobil Research and Engineering Company and FuelCell Energy, Inc.that was in effect on or before the effective filing date of the presentapplication.

Molten carbonate fuel cells utilize hydrogen and/or other fuels togenerate electricity. The hydrogen may be provided by reforming methaneor other reformable fuels in a steam reformer, such as a steam reformerlocated upstream of the fuel cell or integrated within the fuel cell.Fuel can also be reformed in the anode cell in a molten carbonate fuelcell, which can be operated to create conditions that are suitable forreforming fuels in the anode. Still another option can be to performsome reforming both externally and internally to the fuel cell.Reformable fuels can encompass hydrocarbonaceous materials that can bereacted with steam and/or oxygen at elevated temperature and/or pressureto produce a gaseous product that comprises hydrogen.

One of the attractive features of molten carbonate fuel cells is theability to transport CO₂ from a low concentration stream (such as acathode input stream) to a higher concentration stream (such as an anodeoutput flow). During operation, CO₂ and O₂ in an MCFC cathode areconverted to a carbonate ion (CO₃ ²⁻), which is then transported acrossthe molten carbonate electrolyte as a charge carrier. The carbonate ionreacts with H₂ in the fuel cell anode to form H₂O and CO₂. Thus, one ofthe net outcomes of operating the MCFC is transport of CO₂ across theelectrolyte. This transport of CO₂ across the electrolyte can allow anMCFC to generate electrical power while reducing or minimizing the costand/or challenge of sequestering carbon oxides from variousCO_(x)-containing streams. When an MCFC is paired with a combustionsource, such as a natural gas fired power plant, this can allow foradditional power generation while reducing or minimizing the overall CO₂emissions that result from power generation.

U.S. Patent Application Publication 2015/0093665 describes methods foroperating a molten carbonate fuel cell with some combustion in thecathode to provide supplemental heat for performing additional reforming(and/or other endothermic reactions) within the fuel cell anode. Thepublication notes that the voltage and/or power generated by a carbonatefuel cell can start to drop rapidly as the CO₂ concentration falls belowabout 1.0 mole %. The publication further states that as the CO₂concentration drops further, e.g., to below about 0.3 vol %, at somepoint the voltage across the fuel cell can become low enough that littleor no further transport of carbonate may occur and the fuel cell ceasesto function.

An article by Manzolini et al. (Journal of Fuel Cell Science andTechnology, Vol. 9, 2012) describes a method for modeling theperformance of a power generation system using a fuel cell for CO₂separation. Various fuel cell configurations are modeled for processinga CO₂-containing exhaust from a natural gas combined cycle turbine. Thefuel cells are used to generate additional power while alsoconcentrating CO₂ in the anode exhaust of the fuel cells. The lowest CO₂concentration modeled for the cathode outlet of the fuel cells wasroughly 1.4 vol %.

SUMMARY

In an aspect, a method for producing electricity in a molten carbonatefuel cell comprising an electrolyte is provided. The method can includeoperating a molten carbonate fuel cell comprising an anode and a cathodeat a transference of 0.95 or less and an average current density of 60mA/cm² or more, to generate an anode exhaust comprising H₂, CO, and CO₂,and a cathode exhaust comprising 2.0 vol % or less CO₂, 1.0 vol % ormore O₂, and 1.0 vol % or more H₂O.

In another aspect, a method for producing electricity in a moltencarbonate fuel cell comprising an electrolyte is provided. The methodcan include operating a molten carbonate fuel cell comprising an anode,a cathode, and an electrolyte that is more acidic than an electrolytecomposed of (Li_(0.52)Na_(0.48))₂CO₃, at a transference of 0.97 or lessand an average current density of 60 mA/cm² or more, to generate ananode exhaust comprising H₂, CO, and CO₂, and a cathode exhaustcomprising 2.0 vol % or less CO₂, 1.0 vol % or more 02, and 1.0 vol % ormore H₂O.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows an example of a configuration for molten carbonate fuelcells and associated reforming and separation stages.

FIG. 2 shows another example of a configuration for molten carbonatefuel cells and associated reforming and separation stages.

FIG. 3 shows an example of a molten carbonate fuel cell.

FIG. 4 shows a flow pattern example for a molten carbonate fuel cellwith an anode flow direction that is aligned roughly perpendicular to acathode flow direction.

FIG. 5 shows results from operating a fuel cell to cause alternative iontransport with various electrolytes.

DETAILED DESCRIPTION OF THE EMBODIMENTS Overview

In various aspects, molten carbonate fuel cells (MCFCs) are operated toprovide enhanced CO₂ utilization. The enhanced CO₂ utilization isenabled in part by operating an MCFC under conditions that causetransport of alternative ions (different from carbonate ions) across theelectrolyte. The level of alternative ion transport can be characterizedbased on the transference. In some aspects, the enhanced CO₂ utilizationcan be further enhanced based on use of an electrolyte with higheracidity. It has been discovered that higher acidity electrolytes canincrease the amount of CO₂ utilization and/or current density that canbe achieved for a target level of transference. Alternatively, this canreduce the amount of transference that is selected to achieve a targetamount of CO₂ utilization and/or current density.

Conventional operating conditions for molten carbonate fuel cellstypically correspond to conditions where the amount of alternative iontransport is reduced, minimized, or non-existent. The amount ofalternative ion transport can be quantified based on the transferencefor a fuel cell. The transference is defined as the fraction of ionstransported across the molten carbonate electrolyte that corresponds tocarbonate ions, as opposed to hydroxide ions and/or other ions. Aconvenient way to determine the transference can be based on comparinga) the measured change in CO₂ concentration at the cathode inlet versusthe cathode outlet with b) the amount of carbonate ion transportrequired to achieve the current density being produced by the fuel cell.It is noted that this definition for the transference assumes thatback-transport of CO₂ from the anode to the cathode is minimal. It isbelieved that such back-transport is minimal for the operatingconditions described herein. For the CO₂ concentrations, the cathodeinput stream and/or cathode output stream can be sampled, with thesample diverted to a gas chromatograph for determination of the CO₂content. The average current density for the fuel cell can be measuredin any convenient manner.

Under conventional operating conditions, the transference can berelatively close to 1.0, such as 0.98 or more and/or such as havingsubstantially no alternative ion transport. A transference of 0.98 ormore means that 98% or more of the ionic charge transported across theelectrolyte corresponds to carbonate ions. It is noted that hydroxideions have a charge of −1 while carbonate ions have a charge of −2, sotwo hydroxide ions need to be transported across the electrolyte toresult in the same charge transfer as transport of one carbonate ion.

In contrast to conventional operating conditions, operating a moltencarbonate fuel cell with transference of 0.95 or less (or 0.97 or lesswhen operating with a high acidity electrolyte) can increase theeffective amount of carbonate ion transport that is achieved, eventhough a portion of the current density generated by the fuel cell isdue to transport of ions other than carbonate ions. In order to operatea fuel cell with a transference of 0.97 or less, or 0.95 or less,depletion of CO₂ has to occur within the fuel cell cathode. It has beendiscovered that such depletion of CO₂ within the cathode tends to belocalized. As a result, many regions within a fuel cell cathode canstill have sufficient CO₂ for normal operation. These regions containadditional CO₂ that would be desirable to transport across anelectrolyte, such as for carbon capture. However, the CO₂ in suchregions is typically not transported across the electrolyte whenoperating under conventional conditions. By selecting operatingconditions with a transference of 0.97 or less, or 0.95 or less, theregions with sufficient CO₂ can be used to transport additional CO₂while the depleted regions can operate based on alternative iontransport. This can increase the practical limit for the amount of CO₂captured from a cathode input stream.

One difficulty in using MCFCs for elevated CO₂ capture is that theoperation of the fuel cell can potentially be kinetically limited if oneor more of the reactants required for fuel cell operation is present inlow quantities. For example, when using a cathode input stream with aCO₂ content of 4.0 vol % or less, achieving a CO₂ utilization of 75% ormore corresponds to a cathode outlet concentration of 1.0 vol % or less.However, a cathode outlet concentration of 1.0 vol % or less does notnecessarily mean that the CO₂ is evenly distributed throughout thecathode. Instead, the concentration will typically vary within thecathode due to a variety of factors, such as the flow patterns in theanode and the cathode. The variations in CO₂ concentration can result inportions of the cathode where CO₂ concentrations substantially below 1.0vol % are present.

Conventionally, it would be expected that depletion of CO₂ within thecathode would lead to reduced voltage and reduced current density.However, it has been discovered that current density can be maintainedas CO₂ is depleted due to ions other than CO₃ ²⁻ being transportedacross the electrolyte. For example, a portion of the ions transportedacross the electrolyte can correspond to hydroxide ions (OH⁻). Thetransport of alternative ions across the electrolyte can allow a fuelcell to maintain a target current density even though the amount of CO₂transported across the electrolyte is insufficient.

One of the advantages of transport of alternative ions across theelectrolyte is that the fuel cell can continue to operate, even though asufficient number of CO₂ molecules are not kinetically available. Thiscan allow additional CO₂ to be transferred from cathode to anode eventhough the amount of CO₂ present in the cathode would conventionally beconsidered insufficient for normal fuel cell operation. This can allowthe fuel cell to operate with a measured CO₂ utilization closer to 100%,while the calculated CO₂ utilization (based on current density) can beat least 3% greater than the measured CO₂ utilization, or at least 5%greater, or at least 10% greater, or at least 20% greater. It is notedthat alternative ion transport can allow a fuel cell to operate with acurrent density that would correspond to more than 100% calculated CO₂utilization.

Although transport of alternative ions can allow a fuel cell to maintaina target current density, it has further been discovered that transportof alternative ions across the electrolyte can also reduce or minimizethe lifetime of a molten carbonate fuel cell. Thus, mitigation of thisloss in fuel cell lifetime is desirable. It has been unexpectedlydiscovered that increasing the acidity of the electrolyte can reduce orminimize the amount of alternative ions that are transported across theelectrolyte at a fixed set of conditions.

In some aspects, elevated CO₂ capture can be defined based on the amountof transference, such as a transference of 0.97 or less, or 0.95 orless, or 0.93 or less, or 0.91 or less. Maintaining an operatingcondition with transference of 0.97 or less can typically also result ina CO₂ concentration in the cathode output stream of 2.0 vol % or less,or 1.5 vol % or less, or 1.0 vol % or less. At higher CO₂ concentrationsin the cathode output stream, there is typically not sufficient localdepletion of CO₂ to result in lower transference values.

The presence of elevated CO₂ capture can also be indicated by otherfactors, although such other factors are by themselves typically not asufficient condition to indicate elevated CO₂ capture. For example, whenusing a lower CO₂ concentration cathode input stream, elevated CO₂capture can in some aspects correspond to a CO₂ utilization of 70% ormore, or 75% or more, or 80% or more, such as up to 95% or possiblystill higher. Examples of lower concentration sources of CO₂ cancorrespond to CO₂ sources that result in cathode input streamscontaining 5.0 vol % or less of CO₂, or 4.0 vol % or less, such as downto 1.5 vol % or possibly lower. The exhaust from a natural gas turbineis an example of a CO₂-containing stream that often has a CO₂ content of5.0 vol % or less of CO₂, or 4.0 vol % or less. Additionally oralternately, elevated CO₂ capture can correspond to operating conditionswhere the molten carbonate fuel cell is used to generate a substantialamount of current density, such as 60 mA/cm² or more, or 80 mA/cm² ormore, or 100 mA/cm² or more, or 120 mA/cm² or more, or 150 mA/cm² ormore, or 200 mA/cm² or more, such as up to 300 mA/cm² or possibly stillhigher. It is noted that alternative ion transport can also be indicatedby a reduced operating voltage for a fuel cell, as the reaction pathwayfor alternative ion transport has a lower theoretical voltage than thereaction pathway that uses carbonate ions.

Conventionally, the CO₂ concentration in the cathode exhaust of a moltencarbonate fuel cell is maintained at a relatively high value, such as 5vol % CO₂ or more, or 10 vol % CO₂ or more, or possibly still higher.Additionally, molten carbonate fuel cells are typically operated at CO₂utilization values of 70% or less. When either of these conditions arepresent, the dominant mechanism for transport of charge across themolten carbonate electrolyte is transport of carbonate ions. While it ispossible that transport of alternative ions (such as hydroxide ions)across the electrolyte occurs under such conventional conditions, theamount of alternative ion transport is de minimis, corresponding to 2%or less of the current density (or equivalently, a transference of 0.98or more).

As an alternative to describing operating conditions in terms oftransference, the operating conditions can be described based onmeasured CO₂ utilization and “calculated” CO₂ utilization based onaverage current density. In this discussion, the measured CO₂utilization corresponds to the amount of CO₂ that is removed from thecathode input stream. This can be determined, for example, by using gaschromatography to determine the CO₂ concentration in the cathode inputstream and the cathode output stream. This can also be referred to asthe actual CO₂ utilization, or simply as the CO₂ utilization. In thisdiscussion, the calculated CO₂ utilization is defined as the CO₂utilization that would occur if all of the current density generated bythe fuel cell was generated based on transport of CO₃ ²⁻ ions across theelectrolyte (i.e., transport of ions based on CO₂). The difference inmeasured CO₂ utilization and the calculated CO₂ utilization can be usedindividually to characterize the amount of alternative ion transport,and/or these values can be used to calculate the transference, asdescribed above.

In some aspects, any convenient type of electrolyte suitable foroperation of a molten carbonate fuel cell can be used. Many conventionalMCFCs use a eutectic carbonate mixture as the carbonate electrolyte,such as a eutectic mixture of 62 mol % lithium carbonate and 38 mol %potassium carbonate (62% Li₂CO₃/38% K₂CO₃) or a eutectic mixture of 52mol % lithium carbonate and 48 mol % sodium carbonate (52% Li₂CO₃/48%Na₂CO₃). Other eutectic mixtures are also available, such as a eutecticmixture of 40 mol % lithium carbonate and 60 mol % potassium carbonate(40% Li₂CO₃/60% K₂CO₃). While eutectic mixtures of carbonate can beconvenient as an electrolyte for various reasons, non-eutectic mixturesof carbonates can also be suitable. Generally, such non-eutecticmixtures can include various combinations of lithium carbonate, sodiumcarbonate, and/or potassium carbonate. Optionally, lesser amounts ofother metal carbonates can be included in the electrolyte as additives,such as other alkali carbonates (rubidium carbonate, cesium carbonate),or other types of metal carbonates such as barium carbonate, bismuthcarbonate, lanthanum carbonate, or tantalum carbonate. Optionally, themolar amount of lithium in the electrolyte can be less than the combinedamount of one or more other alkali metals in the electrolyte.Optionally, the molar amount of potassium in the electrolyte cancorrespond to 33% or more of the molar amount of alkali metal in theelectrolyte.

For electrolytes corresponding to binary mixtures (such as eutecticmixtures) of alkali carbonates, the acidity of the binary mixtureincreases with increasing average atomic number for the alkali metals inthe mixture. For example, increasing the lithium carbonate content in abinary mixture results in a more basic mixture, while changing fromsodium to potassium at the same molar concentration will result in amore acidic mixture. More generally, the acidity of an alkali carbonateelectrolyte can be determined according to any convenient method.

In some aspects, some of the benefits of operating with substantialalternative ion transport can be achieved at a higher level oftransference, such as a transference of 0.97 or less, by using a higheracidity electrolyte. Additionally or alternately, the amount ofalternative ion transport that is caused by the operating conditions foran MCFC can be reduced or minimized based on the electrolyte present inthe MCFC. In particular, higher acidity electrolytes can reduce orminimize the amount of alternative ion transport relative to loweracidity (i.e., more basic) electrolytes. The exact value of the acidityis not otherwise critical. In some aspects, it can be beneficial to usean electrolyte with a higher acidity than the acidity of a eutecticmixture of 52 mol % lithium carbonate and 48 mol % sodium carbonate (52%Li₂CO₃/48% Na₂CO₃) in order to reduce or minimize alternative iontransport. This can be determined by any convenient method. A variety ofmethods have been applied in the literature to determine basicity, basedon measurement of oxygen solubility in the electrolyte. Such methodsinclude electromotive force, redox equilibria of transition metal ions,and spectroscopic methods. As an example, acidity can be determinedusing galvanic cells to measure Lux-Flood basicity.

It is noted that the structure of a molten carbonate fuel cell can alsohave an impact on how quickly degradation occurs. For example, the openarea of the cathode surface that is available for receiving cathode gascan impact how rapidly degradation occurs. In order to make electricalcontact, at least a portion of the cathode collector is typically incontact with the cathode surface in a molten carbonate fuel cell. Theopen area of a cathode surface (adjacent to the cathode currentcollector) is defined as the percentage of the cathode surface that isnot in contact with the cathode current collector. For conventionalmolten carbonate fuel cell designs, a typical value for the open area isroughly 33%. This is due to the nature of conventional cathode collectorconfigurations, which correspond to a plate-like structure that rests onthe cathode surface, with a portion of the plate-like structure havingopenings that allow for diffusion of cathode gas into the cathode. Invarious aspects, additional benefit can be achieved by using a cathodecurrent collector that provides a larger open area at the cathodesurface, such as an open area of 45% or more, or 50% or more, or 60% ormore, such as up to 90% or possibly still higher.

Conditions for Molten Carbonate Fuel Cell Operation with Alternative IonTransport

In various aspects, the operating conditions for a molten carbonate fuelcell (such as a cell as part of a fuel cell stack) can be selected tocorrespond to a transference of 0.97 or less, or 0.95 or less, therebycausing the cell to transport both carbonate ion and at least one typeof alternative ion across the electrolyte. In addition to transference,operating conditions that can indicate that a molten carbonate fuel cellis operating with transport of alternative ions include, but are notlimited to, CO₂ concentration for the cathode input stream, the CO₂utilization in the cathode, the current density for the fuel cell, thevoltage drop across the cathode, the voltage drop across the anode, andthe O₂ concentration in the cathode input stream. Additionally, theanode input stream and fuel utilization in the anode can be generallyselected to provide the desired current density.

Generally, to cause alternative ion transport, the CO₂ concentration inat least a portion of the cathode needs to be sufficiently low whileoperating the fuel cell to provide a sufficiently high current density.Having a sufficiently low CO₂ concentration in the cathode typicallycorresponds to some combination of a low CO₂ concentration in thecathode input flow, a high CO₂ utilization, and/or a high averagecurrent density. However, such conditions alone are not sufficient toindicate a transference of 0.97 or less, or 0.95 or less.

For example, a molten carbonate fuel cell with a cathode open area ofroughly 33% was operated with a CO₂ cathode inlet concentration of 19vol %, 75% CO₂ utilization, and 160 mA/cm² of average current density.These conditions corresponded to a difference between calculated CO₂utilization and measured CO₂ utilization of less than 1%. Thus, thepresence of substantial alternative ion transport/a transference of 0.97or less, or 0.95 or less, cannot be inferred simply from the presence ofa high CO₂ utilization and a high average current density.

As another example, a molten carbonate fuel cell with a cathode openarea of between 50% and 60% was operated with a CO₂ cathode inletconcentration of 4.0 vol %, 89% CO₂ utilization, and 100 mA/cm² ofcurrent density. These conditions corresponded to a transference of atleast 0.97. Thus, the presence of a transference of 0.95 orless/substantial alternative ion transport cannot be inferred simplyfrom the presence of high CO₂ utilization in combination with low CO₂concentration in the cathode input stream.

As still another example, a molten carbonate fuel cell with a cathodeopen area of between 50% and 60% was operated with a CO₂ cathode inletconcentration of 13 vol %, 68% CO₂ utilization, and 100 mA/cm² ofcurrent density. These conditions corresponded to a transference of atleast 0.98.

In this discussion, operating an MCFC to transport alternative ionsacross the electrolyte is defined as operating the MCFC so that morethan a de minimis amount of alternative ions are transported. It ispossible that minor amounts of alternative ions are transported acrossan MCFC electrolyte under a variety of conventional conditions. Suchalternative ion transport under conventional conditions can correspondto a transference of 0.98 or more, which corresponds to transport ofalternative ions corresponding to less than 2.0% of the current densityfor the fuel cell. By contrast, in this discussion, operating an MCFC tocause alternative ion transport is defined as operating an MCFC with atransference of 0.95 or less, so that 5.0% or more of the currentdensity (or, 5.0% or more of the calculated CO₂ utilization) correspondsto current density based on transport of alternative ions, or 10% ormore, or 20% or more, such as up to 35% or possibly still higher. It isnoted that in some aspects, operating with a high acidity electrolytecan reduce or minimize the amount of alternative ion transport underconditions that would otherwise result in a transference of 0.95 orless. Thus, by operating with a high acidity electrolyte, someconditions with elevated CO₂ capture/substantial alternative iontransport may correspond to a transference of 0.97 or less.

In this discussion, operating an MCFC to cause substantial alternativeion transport (i.e., to operate with a transference of 0.95 or less, or0.97 or less with an electrolyte having high acidity) is further definedto correspond to operating an MCFC with voltage drops across the anodeand cathode that are suitable for power generation. The totalelectrochemical potential difference for the reactions in a moltencarbonate fuel cell is ˜1.04 V. Due to practical considerations, an MCFCis typically operated to generate current at a voltage near 0.7 V orabout 0.8 V. This corresponds to a combined voltage drop across thecathode, electrolyte, and anode of roughly 0.34 V. In order to maintainstable operation, the combined voltage drop across the cathode,electrolyte, and anode can be less than ˜0.5 V, so that the resultingcurrent generated by the fuel cell is at a voltage of 0.55 V or more, or0.6 V or more.

With regard to the anode, one condition for operating with substantialalternative ion transport can be to have an H₂ concentration of 8.0 vol% or more, or 10 vol % or more in the region where the substantialalternative ion transport occurs. Depending on the aspect, this couldcorrespond to a region near the anode inlet, a region near the cathodeoutlet, or a combination thereof. Generally, if the H₂ concentration ina region of the anode is too low, there will be insufficient drivingforce to generate substantial alternative ion transport.

Suitable conditions for the anode can also include providing the anodewith H₂, a reformable fuel, or a combination thereof; and operating withany convenient fuel utilization that generates a desired currentdensity, including fuel utilizations ranging from 20% to 80%. In someaspects this can correspond to a traditional fuel utilization amount,such as a fuel utilization of 60% or more, or 70% or more, such as up to85% or possibly still higher. In other aspects, this can correspond to afuel utilization selected to provide an anode output stream with anelevated content of H₂ and/or an elevated combined content of H₂ and CO(i.e., syngas), such as a fuel utilization of 55% or less, or 50% orless, or 40% or less, such as down to 20% or possibly still lower. TheH₂ content in the anode output stream and/or the combined content of H₂and CO in the anode output stream can be sufficient to allow generationof a desired current density. In some aspects, the H₂ content in theanode output stream can be 3.0 vol % or more, or 5.0 vol % or more, or8.0 vol % or more, such as up to 15 vol % or possibly still higher.Additionally or alternately, the combined amount of H₂ and CO in theanode output stream can be 4.0 vol % or more, or 6.0 vol % or more, or10 vol % or more, such as up to 20 vol % or possibly still higher.Optionally, when the fuel cell is operated with low fuel utilization,the H₂ content in the anode output stream can be in a higher range, suchas an H₂ content of 10 vol % to 25 vol %. In such aspects, the syngascontent of the anode output stream can be correspondingly higher, suchas a combined H₂ and CO content of 15 vol % to 35 vol %. Depending onthe aspect, the anode can be operated to increase the amount ofelectrical energy generated, to increase the amount of chemical energygenerated (i.e., H₂ generated by reforming that is available in theanode output stream), or operated using any other convenient strategythat is compatible with operating the fuel cell to cause alternative iontransport.

In addition to having sufficient H₂ concentration in the anode, one ormore locations within the cathode need to have a low enough CO₂concentration so that the more favorable pathway of carbonate iontransport is not readily available. In some aspects, this can correspondto having a CO₂ concentration in the cathode outlet stream (i.e.,cathode exhaust) of 2.0 vol % or less, or 1.0 vol % or less, or 0.8 vol% or less. It is noted that due to variations within the cathode, anaverage concentration of 2.0 vol % or less (or 1.0 vol % or less, or 0.8vol % or less) in the cathode exhaust can correspond to a still lowerCO₂ concentration in localized regions of the cathode. For example, in across-flow configuration, at a corner of the fuel cell that is adjacentto the anode inlet and the cathode outlet, the CO₂ concentration can belower than a corner of the same fuel cell that is adjacent to the anodeoutlet and the cathode outlet. Similar localized variations in CO₂concentration can also occur in fuel cells having a co-current orcounter-current configuration.

In addition to having a low concentration of CO₂, the localized regionof the cathode can also have 1.0 vol % or more of O₂, or 2.0 vol % ormore. In the fuel cell, O₂ is used to form the hydroxide ion that allowsfor alternative ion transport. If sufficient O₂ is not present, the fuelcell will not operate as both the carbonate ion transport andalternative ion transport mechanisms are dependent on O₂ availability.With regard to O₂ in the cathode input stream, in some aspects this cancorrespond to an oxygen content of 4.0 vol % to 15 vol %, or 6.0 vol %to 10 vol %.

It has been observed that a sufficient amount of water should also bepresent for alternative ion transport to occur, such as 1.0 vol % ormore, or 2.0 vol % or more. Without being bound by any particulartheory, if water is not available in the cathode when attempting tooperate with substantial alternative ion transport, the fuel cellappears to degrade at a much more rapid rate than the deactivation ratethat is observed due to alternative ion transport with sufficient wateravailable. It is noted that because air is commonly used as an O₂source, and since H₂O is one of the products generated duringcombustion, a sufficient amount of water is typically available withinthe cathode.

Due to the non-uniform distribution of cathode gas and/or anode gasduring operation of a molten carbonate fuel cell for elevated CO₂capture, it is believed that one or more of the corners and/or edges ofthe molten carbonate fuel cell will typically have a substantiallyhigher density of alternative ion transport. The one or more corners cancorrespond to locations where the CO₂ concentration in the cathode islower than average, or a location where the H₂ concentration in theanode is greater than average, or a combination thereof.

In this discussion, a fuel cell can correspond to a single cell, with ananode and a cathode separated by an electrolyte. The anode and cathodecan receive input gas flows to facilitate the respective anode andcathode reactions for transporting charge across the electrolyte andgenerating electricity. A fuel cell stack can represent a plurality ofcells in an integrated unit. Although a fuel cell stack can includemultiple fuel cells, the fuel cells can typically be connected inparallel and can function (approximately) as if they collectivelyrepresented a single fuel cell of a larger size. When an input flow isdelivered to the anode or cathode of a fuel cell stack, the fuel cellstack can include flow channels for dividing the input flow between eachof the cells in the stack and flow channels for combining the outputflows from the individual cells. In this discussion, a fuel cell arraycan be used to refer to a plurality of fuel cells (such as a pluralityof fuel cell stacks) that are arranged in series, in parallel, or in anyother convenient manner (e.g., in a combination of series and parallel).A fuel cell array can include one or more stages of fuel cells and/orfuel cell stacks, where the anode/cathode output from a first stage mayserve as the anode/cathode input for a second stage. It is noted thatthe anodes in a fuel cell array do not have to be connected in the sameway as the cathodes in the array. For convenience, the input to thefirst anode stage of a fuel cell array may be referred to as the anodeinput for the array, and the input to the first cathode stage of thefuel cell array may be referred to as the cathode input to the array.Similarly, the output from the final anode/cathode stage may be referredto as the anode/cathode output from the array.

It should be understood that reference to use of a fuel cell hereintypically denotes a “fuel cell stack” composed of individual fuel cells,and more generally refers to the use of one or more fuel cell stacks influid communication. Individual fuel cell elements (plates) cantypically be “stacked” together in a rectangular array called a “fuelcell stack.” This fuel cell stack can typically take a feed stream anddistribute reactants among all of the individual fuel cell elements andcan then collect the products from each of these elements. When viewedas a unit, the fuel cell stack in operation can be taken as a whole eventhough composed of many (often tens or hundreds) of individual fuel cellelements. These individual fuel cell elements can typically have similarvoltages (as the reactant and product concentrations are similar), andthe total power output can result from the summation of all of theelectrical currents in all of the cell elements, when the elements areelectrically connected in series. Stacks can also be arranged in aseries arrangement to produce high voltages. A parallel arrangement canboost the current. If a sufficiently large volume fuel cell stack isavailable to process a given exhaust flow, the systems and methodsdescribed herein can be used with a single molten carbonate fuel cellstack. In other aspects of the invention, a plurality of fuel cellstacks may be desirable or needed for a variety of reasons.

For the purposes of this invention, unless otherwise specified, the term“fuel cell” should be understood to also refer to and/or is defined asincluding a reference to a fuel cell stack composed of a set of one ormore individual fuel cell elements for which there is a single input andoutput, as that is the manner in which fuel cells are typically employedin practice. Similarly, the term fuel cells (plural), unless otherwisespecified, should be understood to also refer to and/or is defined asincluding a plurality of separate fuel cell stacks. In other words, allreferences within this document, unless specifically noted, can referinterchangeably to the operation of a fuel cell stack as a “fuel cell.”For example, the volume of exhaust generated by a commercial scalecombustion generator may be too large for processing by a fuel cell(i.e., a single stack) of conventional size. In order to process thefull exhaust, a plurality of fuel cells (i.e., two or more separate fuelcells or fuel cell stacks) can be arranged in parallel, so that eachfuel cell can process (roughly) an equal portion of the combustionexhaust. Although multiple fuel cells can be used, each fuel cell cantypically be operated in a generally similar manner, given its (roughly)equal portion of the combustion exhaust.

Example of Molten Carbonate Fuel Cell Operation: Cross Flow Orientationfor Cathode and Anode

FIG. 3 shows a general example of a molten carbonate fuel cell. The fuelcell represented in FIG. 3 corresponds to a fuel cell that is part of afuel cell stack. In order to isolate the fuel cell from adjacent fuelcells in the stack, the fuel cell includes separator plates 310 and 311.In FIG. 3, the fuel cell includes an anode 330 and a cathode 350 thatare separated by an electrolyte matrix 340 that contains an electrolyte342. Anode collector 320 provides electrical contact between anode 330and the other anodes in the fuel cell stack, while cathode collector 360provides similar electrical contact between cathode 350 and the othercathodes in the fuel cell stack. Additionally anode collector 320 allowsfor introduction and exhaust of gases from anode 330, while cathodecollector 360 allows for introduction and exhaust of gases from cathode350.

During operation, CO₂ is passed into the cathode collector 360 alongwith O₂. The CO₂ and O₂ diffuse into the porous cathode 350 and travelto a cathode interface region near the boundary of cathode 350 andelectrolyte matrix 340. In the cathode interface region, a portion ofelectrolyte 342 can be present in the pores of cathode 350. The CO₂ andO₂ can be converted near/in the cathode interface region to a carbonateion (CO₃ ²⁻), which can then be transported across electrolyte 342 (andtherefore across electrolyte matrix 340) to facilitate generation ofelectrical current. In aspects where alternative ion transport isoccurring, a portion of the O₂ can be converted to an alternative ion,such as a hydroxide ion or a peroxide ion, for transport in electrolyte342. After transport across the electrolyte 342, the carbonate ion (oralternative ion) can reach an anode interface region near the boundaryof electrolyte matrix 340 and anode 330. The carbonate ion can beconverted back to CO₂ and H₂O in the presence of H₂, releasing electronsthat are used to form the current generated by the fuel cell. The H₂and/or a hydrocarbon suitable for forming H₂ are introduced into anode330 via anode collector 320.

The flow direction within the anode of a molten carbonate fuel cell canhave any convenient orientation relative to the flow direction within acathode. One option can be to use a cross-flow configuration, so thatthe flow direction within the anode is roughly at a 90° angle relativeto the flow direction within the cathode. This type of flowconfiguration can have practical benefits, as using a cross-flowconfiguration can allow the manifolds and/or piping for the anodeinlets/outlets to be located on different sides of a fuel cell stackfrom the manifolds and/or piping for the cathode inlets/outlets.

FIG. 4 schematically shows an example of a top view for a fuel cellcathode operating in a cross-flow configuration, along with arrowsindicating the direction of flow within the fuel cell cathode and thecorresponding fuel cell anode. In FIG. 4, arrow 405 indicates thedirection of flow within cathode 450, while arrow 425 indicates thedirection of flow within the anode (not shown).

Because the anode and cathode flows are oriented at roughly 90° relativeto each other, the anode and cathode flow patterns can contribute tohaving different reaction conditions in various parts of the cathode.The different conditions can be illustrated by considering the reactionconditions in the four corners of the cathode. In the illustration inFIG. 4, the reaction conditions described herein are qualitativelysimilar to the reaction conditions for a fuel cell operating with a CO₂utilization of 70% or more (or 80% or more).

Corner 482 corresponds to a portion of the fuel cell that is close tothe entry point for both the cathode input flow and the anode inputflow. As a result, the concentration of both CO₂ (in the cathode) and H₂(in the anode) is relatively high in corner 482. Based on the highconcentrations, it is expected that portions of the fuel cell nearcorner 482 can operate under expected conditions, with substantially notransport of ions other than carbonate ions across the electrolyte.

Corner 484 corresponds to a portion of the fuel cell that is close tothe entry point for the cathode input flow and close to the exit pointfor the anode output flow. In locations near corner 484, the amount ofcurrent density may be limited due to the reduced concentration of H₂ inthe anode, depending on the fuel utilization. However, sufficient CO₂should be present so that any ions transported across the electrolytesubstantially correspond to carbonate ions.

Corner 486 corresponds to a portion of the fuel cell that is close tothe exit point for the anode output flow and close to the exit point forthe cathode output flow. In locations near corner 486, due to the lowerconcentrations of both H₂ (in the anode) and CO₂ (in the cathode),little or no current would be expected due to the low driving force forthe fuel cell reaction.

Corner 488 corresponds to a portion of the fuel cell that is close tothe entry point for the anode input flow and close to the exit point forthe cathode output flow. The relatively high availability of hydrogen atlocations near corner 488 would be expected to result in substantialcurrent density. However, due to the relatively low concentration ofCO₂, a substantial amount of transport of hydroxide ions and/or otheralternative ions can occur. Depending on the aspect, the substantialamount of alternative ion transport can increase the calculated CO₂utilization by 5% or more, or 10% or more, or 15% or more, or 20% ormore. Additionally or alternately, the transference can be 0.97 or less,or 0.95 or less, or 0.90 or less, or 0.85 or less, or 0.80 or less. Thetransport of substantial amounts of alternative ions across theelectrolyte can temporarily allow higher current densities to bemaintained at locations near corner 488. However, the transport ofalternative ions can also degrade the cathode and/or anode structures,resulting in lower (and possibly no) current density over time atlocations near corner 488. It is noted that at lower amounts ofalternative ion transport (such as a transference of 0.96 or more, or0.98 or more), the amount of lifetime degradation is not as severe.

It has been discovered that when alternative ion transport becomessignificant at one or more locations within the fuel cell, the fuel cellwill quickly begin to degrade. This is believed to be due to the one ormore locations degrading and not providing any further current density.As region(s) stop contributing to the desired current density, theremaining locations in the fuel cell have to operate at higher currentdensities in order to maintain a constant overall (average) currentdensity for the fuel cell. This can cause the region for transport ofalternative ions to grow, resulting in an expanding portion of the fuelcell that degrades and eventually stops working. Alternatively,degradation of a portion of the fuel cell can result in reduced totalcurrent density from the cell, which is also undesirable. The use of anelectrolyte with increased acidity while operating with alternative iontransport can reduce the amount of alternative ion transport thatoccurs, allowing for longer fuel cell lifetimes.

Anode Inputs and Outputs

In various aspects, the anode input stream for an MCFC can includehydrogen, a hydrocarbon such as methane, a hydrocarbonaceous orhydrocarbon-like compound that may contain heteroatoms different from Cand H, or a combination thereof. The source of thehydrogen/hydrocarbon/hydrocarbon-like compounds can be referred to as afuel source. In some aspects, most of the methane (or other hydrocarbon,hydrocarbonaceous, or hydrocarbon-like compound) fed to the anode cantypically be fresh methane. In this description, a fresh fuel such asfresh methane refers to a fuel that is not recycled from another fuelcell process. For example, methane recycled from the anode outlet streamback to the anode inlet may not be considered “fresh” methane, and caninstead be described as reclaimed methane.

The fuel source used can be shared with other components, such as aturbine that uses a portion of the fuel source to provide aCO₂-containing stream for the cathode input. The fuel source input caninclude water in a proportion to the fuel appropriate for reforming thehydrocarbon (or hydrocarbon-like) compound in the reforming section thatgenerates hydrogen. For example, if methane is the fuel input forreforming to generate H₂, the molar ratio of water to fuel can be fromabout one to one to about ten to one, such as at least about two to one.A ratio of four to one or greater is typical for external reforming, butlower values can be typical for internal reforming. To the degree thatH₂ is a portion of the fuel source, in some optional aspects noadditional water may be needed in the fuel, as the oxidation of H₂ atthe anode can tend to produce H₂O that can be used for reforming thefuel. The fuel source can also optionally contain components incidentalto the fuel source (e.g., a natural gas feed can contain some content ofCO₂ as an additional component). For example, a natural gas feed cancontain CO₂, N₂, and/or other inert (noble) gases as additionalcomponents. Optionally, in some aspects the fuel source may also containCO, such as CO from a recycled portion of the anode exhaust. Anadditional or alternate potential source for CO in the fuel into a fuelcell assembly can be CO generated by steam reforming of a hydrocarbonfuel performed on the fuel prior to entering the fuel cell assembly.

More generally, a variety of types of fuel streams may be suitable foruse as an anode input stream for the anode of a molten carbonate fuelcell. Some fuel streams can correspond to streams containinghydrocarbons and/or hydrocarbon-like compounds that may also includeheteroatoms different from C and H. In this discussion, unless otherwisespecified, a reference to a fuel stream containing hydrocarbons for anMCFC anode is defined to include fuel streams containing suchhydrocarbon-like compounds. Examples of hydrocarbon (includinghydrocarbon-like) fuel streams include natural gas, streams containingC1-C4 carbon compounds (such as methane or ethane), and streamscontaining heavier C5+ hydrocarbons (including hydrocarbon-likecompounds), as well as combinations thereof. Still other additional oralternate examples of potential fuel streams for use in an anode inputcan include biogas-type streams, such as methane produced from natural(biological) decomposition of organic material.

In some aspects, a molten carbonate fuel cell can be used to process aninput fuel stream, such as a natural gas and/or hydrocarbon stream, witha low energy content due to the presence of diluent compounds. Forexample, some sources of methane and/or natural gas are sources that caninclude substantial amounts of either CO₂ or other inert molecules, suchas nitrogen, argon, or helium. Due to the presence of elevated amountsof CO₂ and/or inerts, the energy content of a fuel stream based on thesource can be reduced. Using a low energy content fuel for a combustionreaction (such as for powering a combustion-powered turbine) can posedifficulties. However, a molten carbonate fuel cell can generate powerbased on a low energy content fuel source with a reduced or minimalimpact on the efficiency of the fuel cell. The presence of additionalgas volume can require additional heat for raising the temperature ofthe fuel to the temperature for reforming and/or the anode reaction.Additionally, due to the equilibrium nature of the water gas shiftreaction within a fuel cell anode, the presence of additional CO₂ canhave an impact on the relative amounts of H₂ and CO present in the anodeoutput. However, the inert compounds otherwise can have only a minimaldirect impact on the reforming and anode reactions. The amount of CO₂and/or inert compounds in a fuel stream for a molten carbonate fuelcell, when present, can be at least about 1 vol %, such as at leastabout 2 vol %, or at least about 5 vol %, or at least about 10 vol %, orat least about 15 vol %, or at least about 20 vol %, or at least about25 vol %, or at least about 30 vol %, or at least about 35 vol %, or atleast about 40 vol %, or at least about 45 vol %, or at least about 50vol %, or at least about 75 vol %. Additionally or alternately, theamount of CO₂ and/or inert compounds in a fuel stream for a moltencarbonate fuel cell can be about 90 vol % or less, such as about 75 vol% or less, or about 60 vol % or less, or about 50 vol % or less, orabout 40 vol % or less, or about 35 vol % or less.

Yet other examples of potential sources for an anode input stream cancorrespond to refinery and/or other industrial process output streams.For example, coking is a common process in many refineries forconverting heavier compounds to lower boiling ranges. Coking typicallyproduces an off-gas containing a variety of compounds that are gases atroom temperature, including CO and various C₁-C₄ hydrocarbons. Thisoff-gas can be used as at least a portion of an anode input stream.Other refinery off-gas streams can additionally or alternately besuitable for inclusion in an anode input stream, such as light ends(C₁-C₄) generated during cracking or other refinery processes. Stillother suitable refinery streams can additionally or alternately includerefinery streams containing CO or CO₂ that also contain H₂ and/orreformable fuel compounds.

Still other potential sources for an anode input can additionally oralternately include streams with increased water content. For example,an ethanol output stream from an ethanol plant (or another type offermentation process) can include a substantial portion of H₂O prior tofinal distillation. Such H₂O can typically cause only minimal impact onthe operation of a fuel cell. Thus, a fermentation mixture of alcohol(or other fermentation product) and water can be used as at least aportion of an anode input stream.

Biogas, or digester gas, is another additional or alternate potentialsource for an anode input. Biogas may primarily comprise methane and CO₂and is typically produced by the breakdown or digestion of organicmatter. Anaerobic bacteria may be used to digest the organic matter andproduce the biogas. Impurities, such as sulfur-containing compounds, maybe removed from the biogas prior to use as an anode input.

The output stream from an MCFC anode can include H₂O, CO₂, CO, and H₂.Optionally, the anode output stream could also have unreacted fuel (suchas H₂ or CH₄) or inert compounds in the feed as additional outputcomponents. Instead of using this output stream as a fuel source toprovide heat for a reforming reaction or as a combustion fuel forheating the cell, one or more separations can be performed on the anodeoutput stream to separate the CO₂ from the components with potentialvalue as inputs to another process, such as H₂ or CO. The H₂ and/or COcan be used as a syngas for chemical synthesis, as a source of hydrogenfor chemical reaction, and/or as a fuel with reduced greenhouse gasemissions.

The anode exhaust can be subjected to a variety of gas processingoptions, including water-gas shift and separation of the components fromeach other. Two general anode processing schemes are shown in FIGS. 1and 2.

FIG. 1 schematically shows an example of a reaction system for operatinga fuel cell array of molten carbonate fuel cells in conjunction with achemical synthesis process. In FIG. 1, a fuel stream 105 is provided toa reforming stage (or stages) 110 associated with the anode 127 of afuel cell 120, such as a fuel cell that is part of a fuel cell stack ina fuel cell array. The reforming stage 110 associated with fuel cell 120can be internal to a fuel cell assembly. In some optional aspects, anexternal reforming stage (not shown) can also be used to reform aportion of the reformable fuel in an input stream prior to passing theinput stream into a fuel cell assembly. Fuel stream 105 can preferablyinclude a reformable fuel, such as methane, other hydrocarbons, and/orother hydrocarbon-like compounds such as organic compounds containingcarbon-hydrogen bonds. Fuel stream 105 can also optionally contain H₂and/or CO, such as H₂ and/or CO provided by optional anode recyclestream 185. It is noted that anode recycle stream 185 is optional, andthat in many aspects no recycle stream is provided from the anodeexhaust 125 back to anode 127, either directly or indirectly viacombination with fuel stream 105 or reformed fuel stream 115. Afterreforming, the reformed fuel stream 115 can be passed into anode 127 offuel cell 120. A CO₂ and O₂-containing stream 119 can also be passedinto cathode 129. A flow of carbonate ions 122, CO₃ ²⁻, from the cathodeportion 129 of the fuel cell can provide the remaining reactant neededfor the anode fuel cell reactions. Based on the reactions in the anode127, the resulting anode exhaust 125 can include H₂O, CO₂, one or morecomponents corresponding to incompletely reacted fuel (H₂, CO, CH₄, orother components corresponding to a reformable fuel), and optionally oneor more additional nonreactive components, such as N₂ and/or othercontaminants that are part of fuel stream 105. The anode exhaust 125 canthen be passed into one or more separation stages. For example, a CO₂removal stage 140 can correspond to a cryogenic CO₂ removal system, anamine wash stage for removal of acid gases such as CO₂, or anothersuitable type of CO₂ separation stage for separating a CO₂ output stream143 from the anode exhaust. Optionally, the anode exhaust can first bepassed through a water gas shift reactor 130 to convert any CO presentin the anode exhaust (along with some H₂O) into CO₂ and H₂ in anoptionally water gas shifted anode exhaust 135. Depending on the natureof the CO₂ removal stage, a water condensation or removal stage 150 maybe desirable to remove a water output stream 153 from the anode exhaust.Though shown in FIG. 1 after the CO₂ separation stage 140, it mayoptionally be located before the CO₂ separation stage 140 instead.Additionally, an optional membrane separation stage 160 for separationof H₂ can be used to generate a high purity permeate stream 163 of H₂.The resulting retentate stream 166 can then be used as an input to achemical synthesis process. Stream 166 could additionally or alternatelybe shifted in a second water-gas shift reactor 131 to adjust the H₂, CO,and CO₂ content to a different ratio, producing an output stream 168 forfurther use in a chemical synthesis process. In FIG. 1, anode recyclestream 185 is shown as being withdrawn from the retentate stream 166,but the anode recycle stream 185 could additionally or alternately bewithdrawn from other convenient locations in or between the variousseparation stages. The separation stages and shift reactor(s) couldadditionally or alternately be configured in different orders, and/or ina parallel configuration. Finally, a stream with a reduced content ofCO₂ 139 can be generated as an output from cathode 129. For the sake ofsimplicity, various stages of compression and heat addition/removal thatmight be useful in the process, as well as steam addition or removal,are not shown.

As noted above, the various types of separations performed on the anodeexhaust can be performed in any convenient order. FIG. 2 shows anexample of an alternative order for performing separations on an anodeexhaust. In FIG. 2, anode exhaust 125 can be initially passed intoseparation stage 260 for removing a portion 263 of the hydrogen contentfrom the anode exhaust 125. This can allow, for example, reduction ofthe H₂ content of the anode exhaust to provide a retentate 266 with aratio of H₂ to CO closer to 2:1. The ratio of H₂ to CO can then befurther adjusted to achieve a desired value in a water gas shift stage230. The water gas shifted output 235 can then pass through CO₂separation stage 240 and water removal stage 250 to produce an outputstream 275 suitable for use as an input to a desired chemical synthesisprocess. Optionally, output stream 275 could be exposed to an additionalwater gas shift stage (not shown). A portion of output stream 275 canoptionally be recycled (not shown) to the anode input. Of course, stillother combinations and sequencing of separation stages can be used togenerate a stream based on the anode output that has a desiredcomposition. For the sake of simplicity, various stages of compressionand heat addition/removal that might be useful in the process, as wellas steam addition or removal, are not shown.

Cathode Inputs and Outputs

Conventionally, a molten carbonate fuel cell can be operated based ondrawing a desired load while consuming some portion of the fuel in thefuel stream delivered to the anode. The voltage of the fuel cell canthen be determined by the load, fuel input to the anode, air and CO₂provided to the cathode, and the internal resistances of the fuel cell.The CO₂ to the cathode can be conventionally provided in part by usingthe anode exhaust as at least a part of the cathode input stream. Bycontrast, the present invention can use separate/different sources forthe anode input and cathode input. By removing any direct link betweenthe composition of the anode input flow and the cathode input flow,additional options become available for operating the fuel cell, such asto generate excess synthesis gas, to improve capture of carbon dioxide,and/or to improve the total efficiency (electrical plus chemical power)of the fuel cell, among others.

In various aspects, an MCFC can be operated to cause alternative iontransport across the electrolyte for the fuel cell. In some aspectsinvolving low CO₂-content cathode input streams, the CO₂ content of thecathode input stream can be 5.0 vol % or less, or 4.0 vol % or less,such as 1.5 vol % to 5.0 vol %, or 1.5 vol % to 4.0 vol %, or 2.0 vol %to 5.0 vol %, or 2.0 vol % to 4.0 vol %. In other aspects, potentiallyhigher CO₂ concentrations in the cathode input stream can be used, ifthe CO₂ utilization is sufficiently high and/or the current density issufficiently high.

One example of a suitable CO₂-containing stream for use as a cathodeinput flow can be an output or exhaust flow from a combustion source.Examples of combustion sources include, but are not limited to, sourcesbased on combustion of natural gas, combustion of coal, and/orcombustion of other hydrocarbon-type fuels (including biologicallyderived fuels). Additional or alternate sources can include other typesof boilers, fired heaters, furnaces, and/or other types of devices thatburn carbon-containing fuels in order to heat another substance (such aswater or air).

Other potential sources for a cathode input stream can additionally oralternately include sources of bio-produced CO₂. This can include, forexample, CO₂ generated during processing of bio-derived compounds, suchas CO₂ generated during ethanol production. An additional or alternateexample can include CO₂ generated by combustion of a bio-produced fuel,such as combustion of lignocellulose. Still other additional oralternate potential CO₂ sources can correspond to output or exhauststreams from various industrial processes, such as CO₂-containingstreams generated by plants for manufacture of steel, cement, and/orpaper.

Yet another additional or alternate potential source of CO₂ can beCO₂-containing streams from a fuel cell. The CO₂-containing stream froma fuel cell can correspond to a cathode output stream from a differentfuel cell, an anode output stream from a different fuel cell, a recyclestream from the cathode output to the cathode input of a fuel cell,and/or a recycle stream from an anode output to a cathode input of afuel cell. For example, an MCFC operated in standalone mode underconventional conditions can generate a cathode exhaust with a CO₂concentration of at least about 5 vol %. Such a CO₂-containing cathodeexhaust could be used as a cathode input for an MCFC operated accordingto an aspect of the invention. More generally, other types of fuel cellsthat generate a CO₂ output from the cathode exhaust can additionally oralternately be used, as well as other types of CO₂-containing streamsnot generated by a “combustion” reaction and/or by a combustion-poweredgenerator. Optionally but preferably, a CO₂-containing stream fromanother fuel cell can be from another molten carbonate fuel cell. Forexample, for molten carbonate fuel cells connected in series withrespect to the cathodes, the output from the cathode for a first moltencarbonate fuel cell can be used as the input to the cathode for a secondmolten carbonate fuel cell.

In addition to CO₂, a cathode input stream can include O₂ to provide thecomponents necessary for the cathode reaction. Some cathode inputstreams can be based on having air as a component. For example, acombustion exhaust stream can be formed by combusting a hydrocarbon fuelin the presence of air. Such a combustion exhaust stream, or anothertype of cathode input stream having an oxygen content based on inclusionof air, can have an oxygen content of about 20 vol % or less, such asabout 15 vol % or less, or about 10 vol % or less. Additionally oralternately, the oxygen content of the cathode input stream can be atleast about 4 vol %, such as at least about 6 vol %, or at least about 8vol %. More generally, a cathode input stream can have a suitablecontent of oxygen for performing the cathode reaction. In some aspects,this can correspond to an oxygen content of about 5 vol % to about 15vol %, such as from about 7 vol % to about 9 vol %. For many types ofcathode input streams, the combined amount of CO₂ and O₂ can correspondto less than about 21 vol % of the input stream, such as less than about15 vol % of the stream or less than about 10 vol % of the stream. An airstream containing oxygen can be combined with a CO₂ source that has lowoxygen content. For example, the exhaust stream generated by burningcoal may include a low oxygen content that can be mixed with air to forma cathode inlet stream.

In addition to CO₂ and O₂, a cathode input stream can also be composedof inert/non-reactive species such as N₂, H₂O, and other typical oxidant(air) components. For example, for a cathode input derived from anexhaust from a combustion reaction, if air is used as part of theoxidant source for the combustion reaction, the exhaust gas can includetypical components of air such as N₂, H₂O, and other compounds in minoramounts that are present in air. Depending on the nature of the fuelsource for the combustion reaction, additional species present aftercombustion based on the fuel source may include one or more of H₂O,oxides of nitrogen (NOx) and/or sulfur (SOx), and other compounds eitherpresent in the fuel and/or that are partial or complete combustionproducts of compounds present in the fuel, such as CO. These species maybe present in amounts that do not poison the cathode catalyst surfacesthough they may reduce the overall cathode activity. Such reductions inperformance may be acceptable, or species that interact with the cathodecatalyst may be reduced to acceptable levels by known pollutant removaltechnologies.

The amount of O₂ present in a cathode input stream (such as an inputcathode stream based on a combustion exhaust) can advantageously besufficient to provide the oxygen needed for the cathode reaction in thefuel cell. Thus, the volume percentage of O₂ can advantageously be atleast 0.5 times the amount of CO₂ in the exhaust. Optionally, asnecessary, additional air can be added to the cathode input to provide asufficient oxidant for the cathode reaction. When some form of air isused as the oxidant, the amount of N₂ in the cathode exhaust can be atleast about 78 vol %, e.g., at least about 88 vol %, and/or about 95 vol% or less. In some aspects, the cathode input stream can additionally oralternately contain compounds that are generally viewed as contaminants,such as H₂S or NH₃. In other aspects, the cathode input stream can becleaned to reduce or minimize the content of such contaminants.

A suitable temperature for operation of an MCFC can be between about450° C. and about 750° C., such as at least about 500° C., e.g., with aninlet temperature of about 550° C. and an outlet temperature of about625° C. Prior to entering the cathode, heat can be added to or removedfrom the cathode input stream, if desired, e.g., to provide heat forother processes, such as reforming the fuel input for the anode. Forexample, if the source for the cathode input stream is a combustionexhaust stream, the combustion exhaust stream may have a temperaturegreater than a desired temperature for the cathode inlet. In such anaspect, heat can be removed from the combustion exhaust prior to use asthe cathode input stream. Alternatively, the combustion exhaust could beat a very low temperature, for example after a wet gas scrubber on acoal-fired boiler, in which case the combustion exhaust can be belowabout 100° C. Alternatively, the combustion exhaust could be from theexhaust of a gas turbine operated in combined cycle mode, in which thegas can be cooled by raising steam to run a steam turbine for additionalpower generation. In this case, the gas can be below about 50° C. Heatcan be added to a combustion exhaust that is cooler than desired.

Additional Molten Carbonate Fuel Cell Operating Strategies

In some aspects, when operating an MCFC to cause alternative iontransport, the anode of the fuel cell can be operated at a traditionalfuel utilization value of roughly 60% to 80%. When attempting togenerate electrical power, operating the anode of the fuel cell at arelatively high fuel utilization can be beneficial for improvingelectrical efficiency (i.e., electrical energy generated per unit ofchemical energy consumed by the fuel cell).

In some aspects, it may be beneficial to reduce the electricalefficiency of the fuel cell in order to provide other benefits, such asan increase in the amount of H₂ provided in the anode output flow. Thiscan be beneficial, for example, if it is desirable to consume excessheat generated in the fuel cell (or fuel cell stack) by performingadditional reforming and/or performing another endothermic reaction. Forexample, a molten carbonate fuel cell can be operated to provideincreased production of syngas and/or hydrogen. The heat required forperforming the endothermic reforming reaction can be provided by theexothermic electrochemical reaction in the anode for electricitygeneration. Rather than attempting to transport the heat generated bythe exothermic fuel cell reaction(s) away from the fuel cell, thisexcess heat can be used in situ as a heat source for reforming and/oranother endothermic reaction. This can result in more efficient use ofthe heat energy and/or a reduced need for additional external orinternal heat exchange. This efficient production and use of heatenergy, essentially in-situ, can reduce system complexity and componentswhile maintaining advantageous operating conditions. In some aspects,the amount of reforming or other endothermic reaction can be selected tohave an endothermic heat requirement comparable to, or even greaterthan, the amount of excess heat generated by the exothermic reaction(s)rather than significantly less than the heat requirement typicallydescribed in the prior art.

Additionally or alternately, the fuel cell can be operated so that thetemperature differential between the anode inlet and the anode outletcan be negative rather than positive. Thus, instead of having atemperature increase between the anode inlet and the anode outlet, asufficient amount of reforming and/or other endothermic reaction can beperformed to cause the output stream from the anode outlet to be coolerthan the anode inlet temperature. Further additionally or alternately,additional fuel can be supplied to a heater for the fuel cell and/or aninternal reforming stage (or other internal endothermic reaction stage)so that the temperature differential between the anode input and theanode output can be smaller than the expected difference based on therelative demand of the endothermic reaction(s) and the combinedexothermic heat generation of the cathode combustion reaction and theanode reaction for generating electrical power. In aspects wherereforming is used as the endothermic reaction, operating a fuel cell toreform excess fuel can allow for production of increased synthesis gasand/or increased hydrogen relative to conventional fuel cell operationwhile minimizing the system complexity for heat exchange and reforming.The additional synthesis gas and/or additional hydrogen can then be usedin a variety of applications, including chemical synthesis processesand/or collection/repurposing of hydrogen for use as a “clean” fuel.

The amount of heat generated per mole of hydrogen oxidized by theexothermic reaction at the anode can be substantially larger than theamount of heat consumed per mole of hydrogen generated by the reformingreaction. The net reaction for hydrogen in a molten carbonate fuel cell(H₂+½O₂=>H₂O) can have an enthalpy of reaction of about −285 kJ/mol ofhydrogen molecules. At least a portion of this energy can be convertedto electrical energy within the fuel cell. However, the difference(approximately) between the enthalpy of reaction and the electricalenergy produced by the fuel cell can become heat within the fuel cell.This quantity of energy can alternatively be expressed as the currentdensity (current per unit area) for the cell multiplied by thedifference between the theoretical maximum voltage of the fuel cell andthe actual voltage, or <current density>*(Vmax−Vact). This quantity ofenergy is defined as the “waste heat” for a fuel cell. As an example ofreforming, the enthalpy of reforming for methane (CH₄+2H₂O=>4H₂+CO₂) canbe about 250 kJ/mol of methane, or about 62 kJ/mol of hydrogenmolecules. From a heat balance standpoint, each hydrogen moleculeelectrochemically oxidized can generate sufficient heat to generate morethan one hydrogen molecule by reforming. In a conventionalconfiguration, this excess heat can result in a substantial temperaturedifference from anode inlet to anode outlet. Instead of allowing thisexcess heat to be used for increasing the temperature in the fuel cell,the excess heat can be consumed by performing a matching amount of thereforming reaction. The excess heat generated in the anode can besupplemented with the excess heat generated by the combustion reactionin the fuel cell. More generally, the excess heat can be consumed byperforming an endothermic reaction in the fuel cell anode and/or in anendothermic reaction stage heat integrated with the fuel cell.

Depending on the aspect, the amount of reforming and/or otherendothermic reaction can be selected relative to the amount of hydrogenreacted in the anode in order to achieve a desired thermal ratio for thefuel cell. As used herein, the “thermal ratio” is defined as the heatproduced by exothermic reactions in a fuel cell assembly (includingexothermic reactions in both the anode and cathode) divided by theendothermic heat demand of reforming reactions occurring within the fuelcell assembly. Expressed mathematically, the thermal ratio(TH)=Q_(EX)/Q_(EN), where Q_(EX) is the sum of heat produced byexothermic reactions and Q_(EN) is the sum of heat consumed by theendothermic reactions occurring within the fuel cell. Note that the heatproduced by the exothermic reactions can correspond to any heat due toreforming reactions, water gas shift reactions, combustion reactions(i.e., oxidation of fuel compounds) in the cathode, and/or theelectrochemical reactions in the cell. The heat generated by theelectrochemical reactions can be calculated based on the idealelectrochemical potential of the fuel cell reaction across theelectrolyte minus the actual output voltage of the fuel cell. Forexample, the ideal electrochemical potential of the reaction in an MCFCis believed to be about 1.04 V based on the net reaction that occurs inthe cell. During operation of the MCFC, the cell can typically have anoutput voltage less than 1.04 V due to various losses. For example, acommon output/operating voltage can be about 0.7 V. The heat generatedcan be equal to the electrochemical potential of the cell (i.e. ˜1.04 V)minus the operating voltage. For example, the heat produced by theelectrochemical reactions in the cell can be ˜0.34 V when the outputvoltage of ˜0.7 V is attained in the fuel cell. Thus, in this scenario,the electrochemical reactions would produce ˜0.7 V of electricity and˜0.34 V of heat energy. In such an example, the ˜0.7 V of electricalenergy is not included as part of Q_(EX). In other words, heat energy isnot electrical energy.

In various aspects, a thermal ratio can be determined for any convenientfuel cell structure, such as a fuel cell stack, an individual fuel cellwithin a fuel cell stack, a fuel cell stack with an integrated reformingstage, a fuel cell stack with an integrated endothermic reaction stage,or a combination thereof. The thermal ratio may also be calculated fordifferent units within a fuel cell stack, such as an assembly of fuelcells or fuel cell stacks. For example, the thermal ratio may becalculated for a fuel cell (or a plurality of fuel cells) within a fuelcell stack along with integrated reforming stages and/or integratedendothermic reaction stage elements in sufficiently close proximity tothe fuel cell(s) to be integrated from a heat integration standpoint.

From a heat integration standpoint, a characteristic width in a fuelcell stack can be the height of an individual fuel cell stack element.It is noted that the separate reforming stage and/or a separateendothermic reaction stage could have a different height in the stackthan a fuel cell. In such a scenario, the height of a fuel cell elementcan be used as the characteristic height. In this discussion, anintegrated endothermic reaction stage can be defined as a stage heatintegrated with one or more fuel cells, so that the integratedendothermic reaction stage can use the heat from the fuel cells as aheat source for reforming. Such an integrated endothermic reaction stagecan be defined as being positioned less than 10 times the height of astack element from fuel cells providing heat to the integrated stage.For example, an integrated endothermic reaction stage (such as areforming stage) can be positioned less than 10 times the height of astack element from any fuel cells that are heat integrated, or less than8 times the height of a stack element, or less than 5 times the heightof a stack element, or less than 3 times the height of a stack element.In this discussion, an integrated reforming stage and/or integratedendothermic reaction stage that represents an adjacent stack element toa fuel cell element is defined as being about one stack element heightor less away from the adjacent fuel cell element.

A thermal ratio of about 1.3 or less, or about 1.15 or less, or about1.0 or less, or about 0.95 or less, or about 0.90 or less, or about 0.85or less, or about 0.80 or less, or about 0.75 of less, can be lower thanthe thermal ratio typically sought in use of MCFC fuel cells. In aspectsof the invention, the thermal ratio can be reduced to increase and/oroptimize syngas generation, hydrogen generation, generation of anotherproduct via an endothermic reaction, or a combination thereof.

In various aspects of the invention, the operation of the fuel cells canbe characterized based on a thermal ratio. Where fuel cells are operatedto have a desired thermal ratio, a molten carbonate fuel cell can beoperated to have a thermal ratio of about 1.5 or less, for example about1.3 or less, or about 1.15 or less, or about 1.0 or less, or about 0.95or less, or about 0.90 or less, or about 0.85 or less, or about 0.80 orless, or about 0.75 or less. Additionally or alternately, the thermalratio can be at least about 0.25, or at least about 0.35, or at leastabout 0.45, or at least about 0.50. Further additionally or alternately,in some aspects the fuel cell can be operated to have a temperature risebetween anode input and anode output of about 40° C. or less, such asabout 20° C. or less, or about 10° C. or less. Still furtheradditionally or alternately, the fuel cell can be operated to have ananode outlet temperature that is from about 10° C. lower to about 10° C.higher than the temperature of the anode inlet. Yet further additionallyor alternately, the fuel cell can be operated to have an anode inlettemperature greater than the anode outlet temperature, such as at leastabout 5° C. greater, or at least about 10° C. greater, or at least about20° C. greater, or at least about 25° C. greater. Still furtheradditionally or alternately, the fuel cell can be operated to have ananode inlet temperature greater than the anode outlet temperature byabout 100° C. or less, or about 80° C. or less, or about 60° C. or less,or about 50° C. or less, or about 40° C. or less, or about 30° C. orless, or about 20° C. or less.

Operating a fuel cell with a thermal ratio of less than 1 can cause atemperature drop across the fuel cell. In some aspects, the amount ofreforming and/or other endothermic reaction may be limited so that atemperature drop from the anode inlet to the anode outlet can be about100° C. or less, such as about 80° C. or less, or about 60° C. or less,or about 50° C. or less, or about 40° C. or less, or about 30° C. orless, or about 20° C. or less. Limiting the temperature drop from theanode inlet to the anode outlet can be beneficial, for example, formaintaining a sufficient temperature to allow complete or substantiallycomplete conversion of fuels (by reforming) in the anode. In otheraspects, additional heat can be supplied to the fuel cell (such as byheat exchange or combustion of additional fuel) so that the anode inlettemperature is greater than the anode outlet temperature by less thanabout 100° C. or less, such as about 80° C. or less, or about 60° C. orless, or about 50° C. or less, or about 40° C. or less, or about 30° C.or less, or about 20° C. or less, due to a balancing of the heatconsumed by the endothermic reaction and the additional external heatsupplied to the fuel cell.

The amount of reforming can additionally or alternately be dependent onthe availability of a reformable fuel. For example, if the fuel onlycomprised H₂, no reformation would occur because H₂ is already reformedand is not further reformable. The amount of “syngas produced” by a fuelcell can be defined as a difference in the lower heating value (LHV)value of syngas in the anode input versus an LHV value of syngas in theanode output. Syngas produced LHV (sg net)=(LHV(sg out)−LHV(sg in)),where LHV(sg in) and LHV(sg out) refer to the LHV of the syngas in theanode inlet and syngas in the anode outlet streams or flows,respectively. A fuel cell provided with a fuel containing substantialamounts of H₂ can be limited in the amount of potential syngasproduction, since the fuel contains substantial amounts of alreadyreformed H₂, as opposed to containing additional reformable fuel. Thelower heating value is defined as the enthalpy of combustion of a fuelcomponent to vapor phase, fully oxidized products (i.e., vapor phase CO₂and H₂O product). For example, any CO₂ present in an anode input streamdoes not contribute to the fuel content of the anode input, since CO₂ isalready fully oxidized. For this definition, the amount of oxidationoccurring in the anode due to the anode fuel cell reaction is defined asoxidation of H₂ in the anode as part of the electrochemical reaction inthe anode.

An example of a method for operating a fuel cell with a reduced thermalratio can be a method where excess reforming of fuel is performed inorder to balance the generation and consumption of heat in the fuel celland/or consume more heat than is generated. Reforming a reformable fuelto form H₂ and/or CO can be an endothermic process, while the anodeelectrochemical oxidation reaction and the cathode combustionreaction(s) can be exothermic. During conventional fuel cell operation,the amount of reforming needed to supply the feed components for fuelcell operation can typically consume less heat than the amount of heatgenerated by the anode oxidation reaction. For example, conventionaloperation at a fuel utilization of about 70% or about 75% produces athermal ratio substantially greater than 1, such as a thermal ratio ofat least about 1.4 or greater, or 1.5 or greater. As a result, theoutput streams for the fuel cell can be hotter than the input streams.Instead of this type of conventional operation, the amount of fuelreformed in the reforming stages associated with the anode can beincreased. For example, additional fuel can be reformed so that the heatgenerated by the exothermic fuel cell reactions can either be (roughly)balanced by the heat consumed in reforming and/or consume more heat thanis generated. This can result in a substantial excess of hydrogenrelative to the amount oxidized in the anode for electrical powergeneration and result in a thermal ratio of about 1.0 or less, such asabout 0.95 or less, or about 0.90 or less, or about 0.85 or less, orabout 0.80 or less, or about 0.75 or less.

Either hydrogen or syngas can be withdrawn from the anode exhaust as achemical energy output. Hydrogen can be used as a clean fuel withoutgenerating greenhouse gases when it is burned or combusted. Instead, forhydrogen generated by reforming of hydrocarbons (or hydrocarbonaceouscompounds), the CO₂ will have already been “captured” in the anode loop.Additionally, hydrogen can be a valuable input for a variety of refineryprocesses and/or other synthesis processes. Syngas can also be avaluable input for a variety of processes. In addition to having fuelvalue, syngas can be used as a feedstock for producing other highervalue products, such as by using syngas as an input for Fischer-Tropschsynthesis and/or methanol synthesis processes.

In some aspects, the reformable hydrogen content of reformable fuel inthe input stream delivered to the anode and/or to a reforming stageassociated with the anode can be at least about 50% greater than the netamount of hydrogen reacted at the anode, such as at least about 75%greater or at least about 100% greater. Additionally or alternately, thereformable hydrogen content of fuel in the input stream delivered to theanode and/or to a reforming stage associated with the anode can be atleast about 50% greater than the net amount of hydrogen reacted at theanode, such as at least about 75% greater or at least about 100%greater. In various aspects, a ratio of the reformable hydrogen contentof the reformable fuel in the fuel stream relative to an amount ofhydrogen reacted in the anode can be at least about 1.5:1, or at leastabout 2.0:1, or at least about 2.5:1, or at least about 3.0:1.Additionally or alternately, the ratio of reformable hydrogen content ofthe reformable fuel in the fuel stream relative to the amount ofhydrogen reacted in the anode can be about 20:1 or less, such as about15:1 or less or about 10:1 or less. In one aspect, it is contemplatedthat less than 100% of the reformable hydrogen content in the anodeinlet stream can be converted to hydrogen. For example, at least about80% of the reformable hydrogen content in an anode inlet stream can beconverted to hydrogen in the anode and/or in an associated reformingstage(s), such as at least about 85%, or at least about 90%.Additionally or alternately, the amount of reformable fuel delivered tothe anode can be characterized based on the Lower Heating Value (LHV) ofthe reformable fuel relative to the LHV of the hydrogen oxidized in theanode. This can be referred to as a reformable fuel surplus ratio. Invarious aspects, the reformable fuel surplus ratio can be at least about2.0, such as at least about 2.5, or at least about 3.0, or at leastabout 4.0. Additionally or alternately, the reformable fuel surplusratio can be about 25.0 or less, such as about 20.0 or less, or about15.0 or less, or about 10.0 or less.

Example

Molten carbonate fuel cells with electrolytes having various levels ofacidity were operated under conditions suitable for causing alternativeion transport. Fuel cells with three types of electrolytes were used.The electrolytes are listed here in order of decreasingbasicity/increasing acidity. The first electrolyte corresponded to(Li_(0.52)Na_(0.48))₂CO₃. The second electrolyte corresponded to(Li_(0.62)K_(0.38))₂CO₃. The third electrolyte corresponded to(Li_(0.4)K_(0.6))₂CO₃.

The fuel cells corresponded to 250 cm² fuel cells that were operated at650° C. with a current density of 90 mA/cm². Because the fuel cells werebeing operated with varying amounts of alternative ion transport, theoperating voltages for the fuel cells varied between 0.65 and 0.75volts. The anode input stream for each fuel cell included 72 vol % H₂,18 vol % CO₂, and 10 vol % H₂O. The cathode input stream for each fuelcell included 4.0 vol % CO₂, 10 vol % O₂, and 10 vol % H₂O. In order toreduce or minimize the possibility of gas phase mass transfer, thebalance of the cathode input stream corresponded to helium. Thetemperature and current density were maintained while varying the flowrate of the cathode input stream in order to investigate the amount ofalternative ion transport at different levels of actual and/or apparentCO₂ utilization.

The amount of carbonate ion transport versus alternative ion transportwas characterized by using gas chromatography to determine the CO₂concentration in the cathode input stream versus the cathode outputstream for each fuel cell. The gas chromatography values allowed forcalculation of the amount of actual CO₂ utilization in the cathode. Itis noted that all removal of CO₂ from the cathode input stream wasconsidered to correspond to CO₂ utilization in the cathode (i.e., CO₂that was transported across the fuel cell electrolyte as a carbonateion). Based on the amount of carbonate ions transported across theelectrolyte, a current density based on carbonate ion transport could becalculated. The difference between the current density due to carbonateion transport and the measured current density (of roughly 90 mA/cm²)was assumed to correspond to current density due to the transport ofalternative ions across the electrolyte.

FIG. 5 shows the results from operation of the fuel cells with thevarious electrolytes. In FIG. 5, the actual CO₂ utilization asdetermined by gas chromatography is compared with the apparent CO₂utilization based on the current density for the fuel cell. The “parity”line in FIG. 5 corresponds to values where the apparent CO₂ utilizationis the same as the actual CO₂ utilization. As shown in FIG. 5, theapparent CO₂ utilization was greater than 100% under some conditions.

In FIG. 5, the electrolyte with the highest basicity,(Li_(0.52)Na_(0.48))₂CO₃, resulted in the largest amount of alternativeion transport. As the acidity of the electrolyte increased (based on useof (Li_(0.62)K_(0.38))₂CO₃ or (Li_(0.4)K_(0.6))₂CO₃ as the electrolyte),the amount of alternative ion transport substantially and unexpectedlydecreased. For example, in order to achieve roughly 85% actual CO₂utilization with the (Li_(0.52)Na_(0.48))₂CO₃ electrolyte, an apparentCO₂ utilization of roughly 105% was required. This corresponds to havingroughly 20% of the current density correspond to current density basedon alternative ion transport. By contrast, at an apparent CO₂utilization of roughly 103%, the actual CO₂ utilization was close to 90%for the (Li_(0.62)K_(0.38))₂CO₃ electrolyte. This corresponds to havingroughly 15% or less of the current density correspond to current densitybased on alternative ion transport. A still sharper contrast existsrelative to the (Li_(0.4)K_(0.6))₂CO₃ electrolyte, where an actual CO₂utilization of just under 85% was achieved at an apparent CO₂utilization of less than 90%, corresponding to roughly 5% of currentdensity based on alternative ion transport.

ADDITIONAL EMBODIMENTS Embodiment 1

A method for producing electricity in a molten carbonate fuel cellcomprising an electrolyte, the method comprising: operating a moltencarbonate fuel cell comprising an anode and a cathode at a transferenceof 0.95 or less and an average current density of 60 mA/cm² or more, togenerate an anode exhaust comprising H₂, CO, and CO₂, and a cathodeexhaust comprising 2.0 vol % or less CO₂, 1.0 vol % or more O₂, and 1.0vol % or more H₂O.

Embodiment 2

The method of Embodiment 1, wherein the electrolyte is more acidic thanan electrolyte composed of (Li_(0.52)Na_(0.48))₂CO₃.

Embodiment 3

A method for producing electricity in a molten carbonate fuel cellcomprising an electrolyte, the method comprising: operating a moltencarbonate fuel cell comprising an anode, a cathode, and an electrolytethat is more acidic than an electrolyte composed of(Li_(0.52)Na_(0.48))₂CO₃, at a transference of 0.97 or less (or 0.95 orless) and an average current density of 60 mA/cm² or more, to generatean anode exhaust comprising H₂, CO, and CO₂, and a cathode exhaustcomprising 2.0 vol % or less CO₂, 1.0 vol % or more O₂, and 1.0 vol % ormore H₂O.

Embodiment 4

The method of any of the above embodiments, wherein operating the moltencarbonate fuel cell further comprises operating at a measured CO₂utilization of 75% or more, a cathode input stream having a CO₂concentration of 10 vol % or less, or a combination thereof.

Embodiment 5

The method of Embodiment 4, wherein the cathode input stream comprises5.0 vol % or less of CO₂, or wherein the cathode exhaust comprises 1.0vol % or less of CO₂, or a combination thereof.

Embodiment 6

The method of any of the above embodiments, wherein the transference is0.90 or less.

Embodiment 7

The method of any of the above embodiments, wherein the electrolytecomprises Li and one or more additional alkali metals, the electrolytecomprising a greater molar amount of the one or more additional alkalimetals than a molar amount of Li.

Embodiment 8

The method of any of the above embodiments, wherein 33% or more of amolar amount of alkali metal in the electrolyte comprises potassium.

Embodiment 9

The method of any of the above embodiments, wherein the current densityis 100 mA/cm² or more (or 120 mA/cm² or more, or 150 mA/cm² or more).

Embodiment 10

The method of any of the above embodiments, wherein the voltage dropacross the cathode is 0.4 V or less, or wherein the electricity isgenerated at a voltage of 0.55 V or more, or a combination thereof.

Embodiment 11

The method of any of the above embodiments, wherein a fuel utilizationin the anode is 60% or more, or wherein a fuel utilization in the anodeis 55% or less.

Embodiment 12

The method of any of the above embodiments, wherein a H₂ concentrationin the anode exhaust is 5.0 vol % or more, or wherein a combinedconcentration of H₂ and CO in the anode exhaust is 6.0 vol % or more, ora combination thereof.

Embodiment 13

The method of any of the above embodiments, wherein the fuel cell isoperated at a thermal ratio of 0.25 to 1.0.

Embodiment 14

The method of any of the above embodiments, wherein an amount of areformable fuel introduced into the anode, into a reforming elementassociated with the anode, or a combination thereof, is at least about75% greater than the amount of hydrogen reacted in the molten carbonatefuel cell to generate electricity.

Embodiment 15

The method of claim 1, further comprising: introducing an anode inputstream into the anode of a molten carbonate fuel cell; introducing acathode input stream comprising O₂, CO₂, and H₂O into the cathode of themolten carbonate fuel cell.

ALTERNATIVE EMBODIMENTS Alternative Embodiment 1

A method for producing electricity in a molten carbonate fuel cellcomprising an electrolyte, the method comprising: introducing an anodeinput stream (optionally comprising H₂) into an anode of a moltencarbonate fuel cell; introducing a cathode input stream comprising H₂O,O₂, and CO₂ into a cathode of the molten carbonate fuel cell; andoperating the molten carbonate fuel cell at a measured CO₂ utilizationof 70% or more and an average current density of 80 mA/cm² or more togenerate electricity, an anode exhaust comprising H₂, CO, and CO₂, and acathode exhaust comprising CO₂, 1.0 vol % or more O₂, and 1.0 vol % ormore H₂O, wherein a calculated CO₂ utilization calculated based on theaverage current density is greater than the measured CO₂ utilization by5.0% or more.

Alternative Embodiment 2

The method of any of the above alternative embodiments, wherein thecathode input stream comprises 5.0 vol % or less of CO₂ (or 4.0 vol % orless), or wherein the cathode exhaust comprises 1.5 vol % or less of CO₂(or 1.0 vol % or less), or a combination thereof.

Alternative Embodiment 3

A method for producing electricity in a molten carbonate fuel cellcomprising an electrolyte, the method comprising: introducing an anodeinput stream (optionally comprising H₂) into an anode of a moltencarbonate fuel cell; introducing a cathode input stream comprising H₂O,02, and 5.0 vol % or less CO₂ (or 4.0 vol % or less) into a cathode ofthe molten carbonate fuel cell; and operating the molten carbonate fuelcell at an average current density of 80 mA/cm² or more and a measuredCO₂ utilization of 70% or more to generate electricity, an anode exhaustcomprising H₂, CO, and CO₂, and a cathode exhaust comprising 1.0 vol %or less of CO₂, 1.0 vol % or more O₂, and 1.0 vol % or more H₂O, whereina calculated CO₂ utilization calculated based on the average currentdensity is greater than the measured CO₂ utilization.

Alternative Embodiment 4

A method for producing electricity in a molten carbonate fuel cellcomprising an electrolyte, the method comprising: introducing an anodeinput stream (optionally comprising H₂) into an anode of a moltencarbonate fuel cell; introducing a cathode input stream comprising O₂and 5.0 vol % or less CO₂ into a cathode of the molten carbonate fuelcell; and operating the molten carbonate fuel cell at a measured CO₂utilization of 75% or more and an average current density of 150 mA/cm²or more to generate electricity, an anode exhaust comprising H₂, CO, andCO₂, and a cathode exhaust comprising a CO₂ content of 2.0 vol % orless, wherein the CO₂ content of the cathode exhaust is greater than acalculated cathode exhaust CO₂ content based on the current density.

Alternative Embodiment 5

The method of any of the above alternative embodiments, wherein thecalculated CO₂ utilization based on the average current density isgreater than the measured CO₂ utilization by 5.0% or more (or 10% ormore, or 20% or more).

Alternative Embodiment 6

The method of any of the above alternative embodiments, wherein theelectrolyte is more acidic than an electrolyte composed of(Li_(0.52)Na_(0.48))₂CO₃.

Alternative Embodiment 7

The method of any of the above alternative embodiments, wherein theelectrolyte comprises Li and one or more additional alkali metals, theelectrolyte comprising a greater molar amount of the one or moreadditional alkali metals than a molar amount of Li.

Alternative Embodiment 8

The method of any of the above alternative embodiments, wherein 33% ormore of a molar amount of alkali metal in the electrolyte comprisespotassium.

Alternative Embodiment 9

The method of any of the above alternative embodiments, wherein theaverage current density is 100 mA/cm² or more (or 120 mA/cm² or more, or150 mA/cm² or more); or wherein measured first CO₂ utilization is 75% ormore (or 80% or more); or a combination thereof.

Alternative Embodiment 10

The method of any of the above alternative embodiments, wherein thevoltage drop across the cathode is 0.4 V or less, or wherein theelectricity is generated at a voltage of 0.55 V or more, or acombination thereof.

Alternative Embodiment 11

The method of any of the above alternative embodiments, wherein a fuelutilization in the anode is 60% or more, or wherein a fuel utilizationin the anode is 55% or less.

Alternative Embodiment 12

The method of any of the above alternative embodiments, wherein a H₂concentration in the anode exhaust is 5.0 vol % or more, or wherein acombined concentration of H₂ and CO in the anode exhaust is 6.0 vol % ormore, or a combination thereof.

Alternative Embodiment 13

The method of any of the above alternative embodiments, wherein the fuelcell is operated at a thermal ratio of 0.25 to 1.0.

Alternative Embodiment 14

The method of any of the above alternative embodiments, wherein theanode input stream comprises a reformable fuel.

Alternative Embodiment 15

The method of Alternative Embodiment 14, wherein an amount of thereformable fuel introduced into the anode, an internal reforming elementassociated with the anode, or the combination thereof, is at least about75% greater than the amount of hydrogen reacted in the molten carbonatefuel cell to generate electricity.

All numerical values within the detailed description and the claimsherein are modified by “about” or “approximately” the indicated value,and take into account experimental error and variations that would beexpected by a person having ordinary skill in the art.

Although the present invention has been described in terms of specificembodiments, it is not necessarily so limited. Suitablealterations/modifications for operation under specific conditions shouldbe apparent to those skilled in the art. It is therefore intended thatthe following claims be interpreted as covering all suchalterations/modifications that fall within the true spirit/scope of theinvention.

1. A method for producing electricity in a molten carbonate fuel cellcomprising an electrolyte, the method comprising: operating a moltencarbonate fuel cell comprising an anode and a cathode at a transferenceof 0.95 or less and an average current density of 60 mA/cm² or more, togenerate an anode exhaust comprising H₂, CO, and CO₂, and a cathodeexhaust comprising 2.0 vol % or less CO₂, 1.0 vol % or more O₂, and 1.0vol % or more H₂O.
 2. The method of claim 1, wherein operating themolten carbonate fuel cell further comprises operating at a measured CO₂utilization of 75% or more, a cathode input stream having a CO₂concentration of 10 vol % or less, or a combination thereof.
 3. Themethod of claim 2, wherein the cathode input stream comprises 5.0 vol %or less of CO₂, or wherein the cathode exhaust comprises 1.0 vol % orless of CO₂, or a combination thereof.
 4. The method of claim 1, whereinthe transference is 0.90 or less.
 5. The method of claim 1, wherein theelectrolyte is more acidic than an electrolyte composed of(Li_(0.52)Na₀₄₈)₂CO₃.
 6. The method of claim 1, wherein the electrolytecomprises Li and one or more additional alkali metals, the electrolytecomprising a greater molar amount of the one or more additional alkalimetals than a molar amount of Li.
 7. The method of claim 1, wherein 33%or more of a molar amount of alkali metal in the electrolyte comprisespotassium.
 8. The method of claim 1, wherein the current density is 150mA/cm² or more.
 9. The method of claim 1, wherein the voltage dropacross the cathode is 0.4 V or less, or wherein the electricity isgenerated at a voltage of 0.55 V or more, or a combination thereof. 10.The method of claim 1, wherein a fuel utilization in the anode is 60% ormore, or wherein a fuel utilization in the anode is 55% or less.
 11. Themethod of claim 1, wherein a H₂ concentration in the anode exhaust is5.0 vol % or more, or wherein a combined concentration of H₂ and CO inthe anode exhaust is 6.0 vol % or more, or a combination thereof. 12.The method of claim 1, wherein the fuel cell is operated at a thermalratio of 0.25 to 1.0.
 13. The method of claim 1, wherein an amount of areformable fuel introduced into the anode, into a reforming elementassociated with the anode, or a combination thereof, is at least about75% greater than the amount of hydrogen reacted in the molten carbonatefuel cell to generate electricity.
 14. The method of claim 1, furthercomprising: introducing an anode input stream into the anode of a moltencarbonate fuel cell; and introducing a cathode input stream comprisingO₂, CO₂, and H₂O into the cathode of the molten carbonate fuel cell. 15.A method for producing electricity in a molten carbonate fuel cellcomprising an electrolyte, the method comprising: operating a moltencarbonate fuel cell comprising an anode, a cathode, and an electrolytethat is more acidic than an electrolyte composed of(Li_(0.52)Na_(0.48))₂CO₃, at a transference of 0.97 or less and anaverage current density of 60 mA/cm² or more, to generate an anodeexhaust comprising H₂, CO, and CO₂, and a cathode exhaust comprising 2.0vol % or less CO₂, 1.0 vol % or more O₂, and 1.0 vol % or more H₂O. 16.The method of claim 15, wherein the electrolyte comprises Li and one ormore additional alkali metals, the electrolyte comprising a greatermolar amount of the one or more additional alkali metals than a molaramount of Li.
 17. The method of claim 15, wherein 33% or more of a molaramount of alkali metal in the electrolyte comprises potassium.
 18. Themethod of claim 15, wherein operating the molten carbonate fuel cellfurther comprises operating at a measured CO₂ utilization of 75% ormore, a cathode input stream having a CO₂ concentration of 10 vol % orless, or a combination thereof.
 19. The method of claim 18, wherein thecathode input stream comprises 5.0 vol % or less of CO₂, or wherein thecathode exhaust comprises 1.0 vol % or less of CO₂, or a combinationthereof.
 20. The method of claim 15, wherein the transference is 0.90 orless.